Method of heat treating heavy alloy steel forgings



United States Patent 3,264,145 METHOD OF HEAT TREATING HEAVY ALLOY STEEL FORGINGS John E. Steiner, Churchill Borough, Allegheny County,

Pa, assignor to United States Steel Corporation, a corporation of Delaware No Drawing. Filed Sept. 3, 1963, Ser. No. 306,316 2 Claims. (Cl. 148-12.3)

This invention relates to improvements in heat treating heavy alloy steel forgings and more particularly to forgings which are used in the production of large generator and turbine rotors.

Heretofore in the production of alloy steel forgings for generator and turbine rotors the heavy forgings, i.e. forgings over 30 inches in diameter have been subjected to very long and expensive heat treatment to develop required physical and mechanical properties. The steps in the heat treatment have been as follows:

(1) Slowly cooling after forging, usually from a temperature of about 1500 F., at a rate of about F. per hour to about 600 F. and holding at this temperature for about 30 hours.

(2) First austenitizing trezztment.Reheating to an austenitizing temperature of from 1750 to 1850 F. and holding at this temperature for about 30 hours.

(3) First normalizing treatmenL-Cooling (usually in air) to about 600 F. and holding at this temperature for about 30 hours.

(4) Preiemper treatment (optional ).-Heating to a tempering temperature of from 1100 to 1275 F., holding at this temperature for about 30 hours and cooling usually in a furnace to room temperature.

"ice

as copper, aluminum, nitrogen, etc. in small amounts which do not adversely affect the properties.

I have discovered that with such steel a simple heat treatment consisting of equalizing the temperature of the forging in the range of 300 to 600 F. following air cooling from forging temperature; tempering, austenitiz ing, normalizing and tempering will produce results as good or better than the prior art treatment. My treatment is as follows:

(1) Air cool the forging to between about 300 and 650 F. after the forging operation and hold at this tem perature for about 10 hours or at least until the center of the forging is at approximately the same temperature as the surface.

(2) Heat to a tempering temperature of 1225 to 1325 F., hold at this temperature for at least about 30 hours, and air cool to room temperature.

(3) Heat to an austenitizing or normalizing temperature between 1700 and 1850 F. and hold at this temperature for at least about 30 hours (the only austenitizing treatment).

(4) Cool in air by fan, fog, spray or liquid quench to at least 600 F. or lower preferably to room temperature and then hold at a temperature within the range 200 to 600 F. for up to 30 hours (the only normalizing treatment).

(5) Heat to a tempering temperature of 1200 to 1300 F. and hold at this temperature for at least about 30 hours, then furnace cool, i.e. slow cool at a rate not exceeding 30 F. per hour, to room temperature. Machining and testing may be performed after steps 2 and 5. Comparative studies have been conducted on steels of the following chemical composition (percent):

Steel C M11 P S Si Ni Cr Mo V AL N 1 Acid soluble.

N.D.-Not determined.

We have discovered that the foregoing heat treatment time can be reduced from between 400 to 600 hours to between 150 to 300 hours for steels of the following composition, not only without impairment of the properties obtained but with actual improvements therein. The steel composition for which the treatment is effective is as follows:

Percent Carbon .25/.40 Manganese .25/ 1.25 Silicon .40 max. Nickel 1.25 max. Chromium .75/ 1.75 Molybdenum M .90/2.25 Vanadium .20/.70

balance iron, residual impurities and other elements such Tables I and II appearing below give comparative physical property data obtained by single normalizing specimens of steel A in accordance with this invention and of holding at various temperatures as compared with the conventional double normalizing practice.

Single normalizedand tempered (SN & T).Austenitized for 24 hours at 1750 F., cooled at: about F. per hour (approximately the cooling rate at a location 3 inches from the surface of an aincooled 45-inch-diameter rotor forging) to 800 F. or 500 F., held for 24 hours and free furnace cooled to room temperature-or cooled from 1750 F. at about 130 F. per hours to 600 F. and free furnace cooled to room temperature-and tempered for 24 hours at 1230 F. The tension test and stressrupture specimen blanks were free furnace cooled after tempering, whereas the impact rest blanks were either brine quenched (BQ) (unembrittled condition) or slowly cooled (SC) at about 7 F. per hour to 700 F. and then free furnace cooled to room temperature.

Double normalized and tempered (DN & T .+-Austenitized for 4 hours at 1850 F. andfree furnace cooled to room temperature, then heat treated as described under the single normalized and tempered treatment described Table I shows the longitudinal mechanical properties obtained for steel A on 0.505-inch-diameter tension-test specimens tested at room temperature, Charpy V-notch impact test specimens tested at various temperatures, and combination smooth-notched 0.252-inch-diameter stressrupture-test specimens tested at various temperatures with various applied stresses.

T able I Heat Treatment SN 15: T DN & T SN &'T DN & T SN & T DN & T

Holdin Tern erature after Normalizing F. 24 Hr. 800 800 500 500 75 75 Yield Strength .5.

0.02% Offset 116, 500 117, 400 114, 600 117, 200 118, 200 115, 500 0.2% Ofiset c 128, 500 128, 100 124, 400 130, 400 131, 000 128, 800 Tensile Strength p.s.i 143,300 144,100 141,600 146,900 148,300 144,000 Elongation in 2 Inches, percen 16.0 15.0 15. 5 11. 5 13. 12.0 Reduction of area, percent 55. 48. 4 51. 9 32. 2 41. 8 44.0 Energy Absorbed at Room Temperature,

Table II 100 F., 60,000 p.s.1. 1,000 F., 50,000 p.s.i. 1,100 F., 40,000 p.s.i.

T.R. Elong 14 IRA. T.R. Elong. R.A. 'I.R. Elong. RA.

T 00 17.3 77.9 69. 7 20.0 84.3 SN & 8 15. 3 76.0 157. 4 21. 3 81.1 24. 0 79. 5 18. 5 26. 6 87. 7 17. 4 69. 3 186. 7 16. 6 75. 0 14. 6 40. 0 200. 4 22. 6 82. 9 15. 3 54. 0 157. 8 18. 0 80. 8

1,100 F., 30,000 p.s.i. 1,200 F., 20,000 p.s.i. 1,250 F., 15,000 p.s.i.

T.R. Elong R.A. T.R. Elong. RA. T.R. Elong. R.A.

. 20. 6 77. 6 209. 0 34. 2 91. 3 68. 3 30. 6 91. 8 17. 3 69. 2 275. 5 16. 6 85. 9 135. 6 25. 3 93. 1 24. 0 85. 0 134. 8 26. 8 90. 9 108. 0 32. 6 93.3 20. 0 79. 2 174. 2 30. 8 88. 8 69. 4 28. 0 92. 4 19. 3 80. 9 236. 8 22. 0 89. 3 108. 1 38. 0 92. 7 DN 8: T75-- 1, 136. 3 23. 3 74.0 279. 7 24. 0 85. 9 137. 8 33. 3 91.8

1 Time to rupture in hours. 2 Percent elongation in 1 inch. 3 Percent reduction of area.

The data of Tables I and II show that the mechanical properties of the steels contemplated by this invention are only slightly affected by the variations in heat treatment investigated and show that single normalizing from 1750 F. directly to room temperature (75 F.) rather than double normalizing can be safely employed for heavy forgings.

To further show the effect on mechanical properties of single normalizing (from 1775 F.) versus double normalizing (from 1850 F. and then from 1775 F., or from 1850 F. and then from 1745 F.), and of cooling after final normalizing at rates in the range 90 F. per hour to 130 F. per hour, and of tempering at various temperatures in the range of 1225 F. to 1260 F., specimens of steels B and C were 'heat treated as follows and tested.

Single normalized and tempered. Samples of steels B and C were austenitized for 4 hours at 1775 F and sammples of steel B cooled ata'b out 90 F. per hour and samples of steel C about 130 F. per hour to 600 R, free furnace cooled to room temperature and tempered for 24 hours at the temperatures indicated in the Tables II and III. After tempering, the tension test and stress-rupture specimen blanks were free furnace cooled to room temperature, whereas the impact-test blanks were either brinequenched (BQ) (embrittled condition) or slowly cooled (SC) at about 7 F. per hour to 750 F. and then free furnace cooled to room temperature.

Double normalized and tempered. Samples of steels B and C were austenitized for 4 hours at 1850" F. and free furnace cooled to room temperature. Samples of steel B were re-austenitized for 4 hours at 1775 F. and steel C at 1745" F., and then cooled and subsequently single normalized and tempered heat treated as described immediately above. The following Tables III and IV show the longitudinal mechanical properties obtained for steels B and C on 0.505-inch-diameter tension-test specimens tested at room temperature, Charpy V-notch impact-test specimens tested at various temperatures, and combination smooth-notched 0.357-inch-diameter stress-rupture test specimens (steel B) or combination smooth-notched 0.252-inch-diameter stress-rupture test specimens (steel C) tested at various temperatures with various applied stresses.

Table III Steel B B C C Heat Treatment SN & T DN & '1 SN & T DN & T DN & T

Tampering Temperature, F 1, 225 1, 225 1, 200 1, 240 1, 250 Yield Strength, p.s.i.:

0.02% OfiseL. 75, 900+ 88, 000 101, 900 93, 400 95,000

0.2 Offset... 73,300+ 89,000 104, 000 97,000 96, 000 Tensile Strength, p.s 98,000 111, 900 124, 600 119, 800 118, 800 Elongation in 2 Inches, Percent 23. 21.0 17. 0 18. 5 20. 0 Reduction of Area, Percent 61.0 62. 2 56. 4 57. 8 62. 0 Ezfierfiy Absorbed at Room Temperature,

(BQ) 9.0 8. 0 10. 0 8. 5 ND C) 10. 5 7. 5 9. 5 4. 5 8. 5

%, Shear Fracture-Transition Temperature,

lBQ) 195 205 195 205 ND 1 Double normalized from 1,850 F. and from 1,745 F. All other DN & '1 specimens were double normalized from 1,850 F. and from 1,775 F.

ND-Not determined.

Table IV 1,000 F., 00,000 p.s.i. 1,000 F., 50,000 p.s.i. Steel Treatment Tempering Temp, F.

T.R. 1 Elong. 1 RA. 3 T.R. Elong RA.

1,100 F., 40,000 psi 1,100 F 30,000 p s 1 TR. Elong RA. T.R. Elong. RA.

1,200 F., 20,000 p.s.i. 1,250 1 15,000 p.s.i.

T.R. Elong R.A. I.R. Elong. R.A.

B SN & T 1, 225 67.0 13. 0 92. 1 47. 9 67.0 93. 6 B DN & T 1, 225 75. 4 12.0 92. 2

164. 1 10.0 88. 9 67.1 80.0 92. 5 C SN & 'I 1, 260 331.4 22.9 81. 7 192. 7 22. 2 83, 4 C DN & T 1, 240 266. 0 24. 1 31. 8 178. 4 24. 8 89.1

1 Time to rupture in hours. 1 Percent elongation in 1 inch. 3 Percent reduction of area.

The data of Tables III and IV show that not only are 50 Molybdenum .90/ 2.25 none of the mechanical properties of these three chro- Vanadium .20/ .70

mium-molybdenum-vanadium steels impaired by the single normalizing of this invention instead of conventional double normalizing but are in many respects improved. For example, the notch toughness of steel C and the stressrupture properties of steel C were significantly better after the single normalizing treatment than after the double normalizing treatment. All the specimens were cooled directly to room temperature during normalizing.

While I have shown and described several specific embodiments of my invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of my invention, as defined in the appended claims.

I claim:

1. In the production of large rotor forgings, the method comprising forging a rotor from steel containing essentially iron and Percent Carbon .25/ .40 Manganese .25/1.25 Silicon .40 max.

Nickel 1.25 max. Chromium .75/ 1.75

air cooling the forging to between 300 and 650 F. and hold at this temperature for at least sufficient time to bring the 'center of the forging to approximately the same temperature as the surface, heating the forging to a tempering temperature of 1225 to 1325 F. and air cool, heating to an austenitizing temperature of about 1750 F. and hold at such temperature for at least about 30 hours, air cool to at least 600 F. and hold at a temperature between 200 and 600 F. for up to 30 hours and then beat to a tempering temperature of 1200 to 1300 F., hold at such temperature for at least 30 hours and thereafter slow cool to room temperature.

2. In the production of rotor for-gings, the method com prising forging a rotor from steel consisting of Percent Carbon .25/.40 Manganese .25/1.25 Silicon .40 max. Nickel 1.25 max. Chromium .75/ 1.75 Molybdenum .90/2.25 Vanadium .20/.70

with the balance iron and residual impurities and other elements such as copper, aluminum, nitrogen in small quantities which do not adversely aifect the properties, air cooling the forging to between 300 and 650 F. and hold at this temperature for about 10 hours, then heat the fo-rg ing to a tempering temperature of 1225 to 1325 F. and air cool, then heat to a normalizing temperature of about 1750 F. and hold at such temperature for at least about 30 hours, air cool to room temperature and hold at a temperature between 200 and 600 F. for up to 30 hours No references cited.

HYLAND BIZOT, Primary Examiner.

H. F. SAITO, Assistant Examiner. 

1. IN THE PRODUCTION OF LARGE ROTOR FORGINGS, THE METHOD COMPRISING FOGING A ROTOR FROM STEEL CONTAINING ESSENTAILLY IRON AND PERCENT CARBON 25/.40 MANGANESE .25/1.25 SILICON .40 MAX. NICKEL 1.25 MAX. CHROMIUM .75/1.75 MOLYBDENUM .90/2.25 VANADIUM .20/70 AIR COOLING THE FORGING TO BETWEEN 300 AND 650*F. AND HOLD AT THIS TEMPERATURE FOR AT LEAST SUFFICIENT TIME TO BRING THE CENTER OF THE FORGING TO APPROXIMATELY THE SAME TEMPERATURE AS THE SURFACE HEATING THE FORGING TO A TEMPERING TEMPERATUE OF 1225 TO 1325*F. AND AIR COOL, HEATING TO AN AUSTENITIZING TEMPERATURE OF ABOUT 1750*F. AND HOLD AT SUCH TEMPERATURE FOR AT LEAST ABOUT 30 HOURS, AIR COOL TO AT LEAST 600*F. AND HOLD AT A TEMPERATURE BETWEEN 200 AND 600*F. FOR UP TO 30 HOURS AND THEN HEAT TO A TEMPERING TEMPERATURE OF 1200 TO 1300*F., HOLD AT SUCH TEMPERATURE FOR AT LEAST 30 HOURS AND THEREAFTER SLOW COOL TO ROOM TEMPERATUE. 